Semiconductor Structure having Nanocrystalline Core and Nanocrystalline Shell Pairing with Compositional Transition Layer

ABSTRACT

Semiconductor structures having a nanocrystalline core and nanocrystalline shell pairing compositional transition layers are described. In an example, a semiconductor structure includes a nanocrystalline core composed of a first semiconductor material. A nanocrystalline shell composed of a second semiconductor material surrounds the nanocrystalline core. A compositional transition layer is disposed between, and in contact with, the nanocrystalline core and nanocrystalline shell and has a composition intermediate to the first and second semiconductor materials. In another example, a semiconductor structure includes a nanocrystalline core composed of a first semiconductor material. A nanocrystalline shell composed of a second semiconductor material surrounds the nanocrystalline core. A nanocrystalline outer shell surrounds the nanocrystalline shell and is composed of a third semiconductor material. A compositional transition layer is disposed between, and in contact with, the nanocrystalline shell and the nanocrystalline outer shell and has a composition intermediate to the second and third semiconductor materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/663,300, filed on Oct. 29, 2012, the entire contents of which arehereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the present invention are in the field of quantum dotsand, in particular, semiconductor structures having a nanocrystallinecore and nanocrystalline shell pairing with one or more compositionaltransition layers.

BACKGROUND

Quantum dots having a high photoluminescence quantum yield (PLQY) may beapplicable as down-converting materials in down-convertingnanocomposites used in solid state lighting applications.Down-converting materials are used to improve the performance,efficiency and color choice in lighting applications, particularly lightemitting diodes (LEDs). In such applications, quantum dots absorb lightof a particular first (available or selected) wavelength, usually blue,and then emit light at a second wavelength, usually red or green.

SUMMARY

Embodiments of the present invention include semiconductor structureshaving a nanocrystalline core and nanocrystalline shell pairing with oneor more compositional transition layers.

In an embodiment, a semiconductor structure includes a nanocrystallinecore composed of a first semiconductor material. A nanocrystalline shellcomposed of a second, different, semiconductor material at leastpartially surrounds the nanocrystalline core. A compositional transitionlayer is disposed between, and in contact with, the nanocrystalline coreand nanocrystalline shell. The compositional transition layer has acomposition intermediate to the first and second semiconductormaterials.

In another embodiment, a semiconductor structure includes ananocrystalline core composed of a first semiconductor material. Ananocrystalline shell composed of a second, different, semiconductormaterial at least partially surrounds the nanocrystalline core. Ananocrystalline outer shell at least partially surrounds thenanocrystalline shell and is composed of a third semiconductor materialdifferent from the first and second semiconductor materials. Acompositional transition layer is disposed between, and in contact with,the nanocrystalline shell and the nanocrystalline outer shell. Thecompositional transition layer has a composition intermediate to thesecond and third semiconductor materials.

In another embodiment, a composite includes a matrix material and aplurality of semiconductor structures embedded in the matrix material.Each semiconductor structure includes a nanocrystalline core composed ofa first semiconductor material. A nanocrystalline shell composed of asecond, different, semiconductor material at least partially surroundsthe nanocrystalline core. A compositional transition layer is disposedbetween, and in contact with, the nanocrystalline core andnanocrystalline shell. The compositional transition layer has acomposition intermediate to the first and second semiconductormaterials. An amorphous insulator coating surrounds and encapsulates thenanocrystalline core/nanocrystalline shell pairing.

In another embodiment, a composite includes a matrix material and aplurality of semiconductor structures embedded in the matrix material.Each semiconductor structure includes a nanocrystalline core composed ofa first semiconductor material. A nanocrystalline shell composed of asecond, different, semiconductor material at least partially surroundsthe nanocrystalline core. A nanocrystalline outer shell at leastpartially surrounds the nanocrystalline shell and is composed of a thirdsemiconductor material different from the first and second semiconductormaterials. A compositional transition layer is disposed between, and incontact with, the nanocrystalline shell and the nanocrystalline outershell. The compositional transition layer has a composition intermediateto the second and third semiconductor materials. An amorphous insulatorcoating surrounds and encapsulates the nanocrystallinecore/nanocrystalline shell/nanocrystalline outer shell combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a plot of prior art core/shell absorption (left y-axis)and emission spectra intensity (right y-axis) as a function ofwavelength for conventional quantum dots.

FIG. 2 illustrates a schematic of a cross-sectional view of a quantumdot, in accordance with an embodiment of the present invention.

FIG. 3 illustrates a schematic of an integrating sphere for measuringabsolute photoluminescence quantum yield, in accordance with anembodiment of the present invention.

FIG. 4 is a plot of photon counts as a function of wavelength innanometers for sample and reference emission spectra used in themeasurement of photoluminescence quantum yield, in accordance with anembodiment of the present invention.

FIG. 5 is a plot including a UV-Vis absorbance spectrum andphotoluminescent emission spectrum for red CdSe/CdS core/shell quantumdots, in accordance with an embodiment of the present invention.

FIG. 6 is a plot including a UV-Vis absorbance spectrum andphotoluminescent emission spectrum for a green CdSe/CdS core/shellquantum dot, in accordance with an embodiment of the present invention.

FIG. 7 illustrates operations in a reverse micelle approach to coating asemiconductor structure, in accordance with an embodiment of the presentinvention.

FIG. 8 is a transmission electron microscope (TEM) image of silicacoated CdSe/CdS core/shell quantum dots having complete silicaencapsulation, in accordance with an embodiment of the presentinvention.

FIGS. 9A-9C illustrate schematic representations of possible compositecompositions for quantum dot integration, in accordance with anembodiment of the present invention.

FIG. 10 is a transmission electron microscope (TEM) image of a sample ofcore/shell CdSe/CdS quantum dots, in accordance with an embodiment ofthe present invention.

FIG. 11 is a plot including a UV-Vis absorbance spectrum andphotoluminescent emission spectrum for a CdSe/CdS core/shell quantum dothaving a PLQY of 96%, in accordance with an embodiment of the presentinvention.

FIG. 12 is a transmission electron microscope (TEM) image of a sample ofCdSe/CdS quantum dots having a PLQY of 96%, in accordance with anembodiment of the present invention.

FIG. 13 illustrates a cross-sectional view of a semiconductor structurehaving a nanocrystalline core and nanocrystalline shell pairing with onecompositional transition layer, in accordance with an embodiment of thepresent invention.

FIG. 14 illustrates a cross-sectional view of a semiconductor structurehaving a nanocrystalline core/nanocrystalline shell/nanocrystallineouter shell combination with two compositional transition layers, inaccordance with an embodiment of the present invention.

FIG. 15 illustrates a cross-sectional view of a semiconductor structurehaving a nanocrystalline core/nanocrystalline shell/nanocrystallineouter shell combination with one compositional transition layer, inaccordance with an embodiment of the present invention.

FIG. 16 is a plot of increasing photoluminescence (PL) intensity as afunction of time (seconds) for non-alloyed rod materials, in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION

Semiconductor structures having a nanocrystalline core andnanocrystalline shell pairing with one or more compositional transitionlayers are described herein. In the following description, numerousspecific details are set forth, such as specific quantum dot geometriesand efficiencies, in order to provide a thorough understanding ofembodiments of the present invention. It will be apparent to one skilledin the art that embodiments of the present invention may be practicedwithout these specific details. In other instances, well-known relatedapparatuses, such as the host of varieties of applicable light emittingdiodes (LEDs), are not described in detail in order to not unnecessarilyobscure embodiments of the present invention. Furthermore, it is to beunderstood that the various embodiments shown in the figures areillustrative representations and are not necessarily drawn to scale.

Also disclosed herein are quantum dots having high photoluminescencequantum yields (PLQY's) and methods of making and encapsulating suchquantum dots. A high PLQY is achieved by using a synthetic process thatsignificantly reduces the defects and self absorption found in prior artquantum dots. The resulting geometries of the quantum dots may includenon-spherical quantum dot cores shelled with a rod-shaped shell. Theaspect or volume ratio of the core/shell pairing may be controlled bymonitoring the reaction process used to fabricate the pairing. Uses ofquantum dot compositions having high PLQYs are also disclosed, includingsolid state lighting. Other applications include biological imaging andfabrication of photovoltaic devices.

As a reference point, quantum dots based on a spherical cadmium selenide(CdSe) core embedded in a cadmium sulfide (CdS) nanorod shell have beenreported. Such quantum dots do not have a high PLQY. Typically, priorart core/shell quantum dots suffer from several structural deficiencieswhich may contribute to a reduced PLQY. For example, prior artcore/shell quantum dots used for down-shifting applications typicallyhave overlapping absorption and emission profiles. Profile overlap maybe attributed to core material selection such that both the absorptionand emission of the quantum dot is controlled by the size, shape, andcomposition of the core quantum dot, and the shell, if any, is used onlyas a passivating layer for the surface. However, the prior artarrangement leads to a significant amount of self-absorption(re-absorption of the down-shifted light), which decreases the measuredPLQY. Accordingly, a typical prior art core/shell quantum dot PLQY isbelow 80% which is often not high enough for device applications. Also,prior art core/shell quantum dots suffer from self absorption due inpart to inappropriate volume of core/shell material.

As an example, FIG. 1 depicts a plot 100 of prior art core/shellabsorption and emission spectra intensity as a function of wavelengthfor conventional quantum dots. The absorption spectra (102 a, 102 b, 102c) are of CdSe core nanorods for a same core size with differentthickness shells (a, b, c). FIG. 1 also depicts the emission spectra(104 a, 104 b, 104 c) of the three core/shell quantum dots afterexposure to blue or UV light. The absorption spectrum and the emissionspectrum overlap for each thickness of shell.

The low PLQY of prior art quantum dots is also attributed to poornanocrystal surface and crystalline quality. The poor quality may resultfrom a previous lack of capability in synthetic techniques for treatingor tailoring the nanocrystal surface in order to achieve PLQYs above 90percent. For example, the surface may have a large number of danglingbonds which act as trap states to reduce emission and, hence, PLQY.Previous approaches to address such issues have included use of a verythin shell, e.g., approximately ½ monolayer to 5 monolayers, or up toabout 1.5 nm of thickness, to preserve the epitaxial nature of theshell. However, a PLQY of only 50-80% has been achieved. In suchsystems, considerable self-absorption may remain, decreasing the PLQY inmany device applications. Other approaches have included attempts togrow a very large volume of up to 19 monolayers, or about 6 nm of shellmaterial on a nanometer-sized quantum dot. However, the results havebeen less than satisfactory due to mismatched lattice constants betweenthe core and shell material.

Conventionally, a spherical shell is grown on a spherical core in orderto fabricate a core/shell quantum dot system. However, if too muchvolume of shell material is added to the core, the shell often will tocrack due to strain. The strain introduces defects and decreases thePLQY. Band-edge emission from the quantum dots is then left to competewith both radiative and non-radiative decay channels, originating fromdefect electronic states. Attempts have been made to use an organicmolecule as a passivating agent in order to improve the size-dependentband-edge luminescence efficiency, while preserving the solubility andprocessability of the particles. Unfortunately, however, passivation byway of organic molecule passivation is often incomplete or reversible,exposing some regions of the surface of a quantum dot to degradationeffects such as photo-oxidation. In some cases, chemical degradation ofthe ligand molecule itself or its exchange with other ligands results infabrication of poor quality quantum dots.

One or more embodiments of the present invention address at least one ormore of the above issues regarding quantum dot quality and behavior andthe impact on PLQY of the fabricated quantum dots. In one approach, thequality of quantum dot particle interfaces is improved over conventionalsystems. For example, in one embodiment, high PLQY and temperaturestability of a fabricated (e.g., grown) quantum dot is centered on thepassivation or elimination of internal (at the seed/rod interface) andexternal (at the rod surface) interface defects that providenon-radiative recombination pathways for electron-hole pairs thatotherwise compete with a desirable radiative recombination. Thisapproach may be generally coincident with maximizing theroom-temperature PLQY of the quantum dot particles. Thus, thermal escapepaths from the quantum dot, assisted by quantum dot photons, aremitigated as a primary escape mechanism for thermally excited carriers.Although the chemical or physical nature of such trap states has notbeen phenomenologically explored, suitably tuning electron density atthe surface may deactivate trap states. Such passivation is especiallyimportant at increased temperatures, where carriers have sufficientthermal energy to access a larger manifold of these states.

In an embodiment, approaches described herein exploit the concept oftrap state deactivation. Furthermore, maintenance of such a deactivationeffect over time is achieved by insulating a quantum dot interfaceand/or outer most surface from an external environment. The deactivationof surface states is also important for the fabrication of polymercomposites including quantum dots, particularly in the case where thepolymer composite is exposed to a high flux light-source (as is the casefor solid-state lighting, SSL) where it is possible for some of theparticles to have more than one exciton. The multi-excitons mayrecombine radiatively or non-radiatively via Auger recombination to asingle exciton state. For non-passivated quantum dot systems, the Augerrate increases with particle volume and with exciton population.However, in an embodiment, a thick, high quality, asymmetric shell of(e.g., of CdS) is grown on well-formed seeds (e.g., CdSe) to mitigateAuger rate increase.

One or more embodiments described herein involve an optimized synthesisof core/shell quantum dots. In a specific example, high PLQY andtemperature stable quantum dots are fabricated from CdSe/CdS core-shellnanorods. In order to optimize the quantum dots in place of lightemitting diode (LED) phosphors, the temperature stability of the quantumdots is enhanced, and the overall PLQY increased. Such improvedperformance is achieved while maintaining high absorption and narrowemission profiles for the quantum dots. In one such embodiment,materials systems described herein are tailored for separateoptimization of absorption and emission by employing a core/shellstructure. The core material predominantly controls the emission and theshell material predominantly controls the absorption. The describedsystems enable separate optimization of absorption and emission andprovides very little overlap of the absorption and emission to minimizere-absorption of any emitted light by the quantum dot material (i.e.,self-absorption).

Several factors may be intertwined for establishing an optimizedgeometry for a quantum dot having a nanocrystalline core andnanocrystalline shell pairing. As a reference, FIG. 2 illustrates aschematic of a cross-sectional view of a quantum dot, in accordance withan embodiment of the present invention. Referring to FIG. 2, asemiconductor structure (e.g., a quantum dot structure) 200 includes ananocrystalline core 202 surrounded by a nanocrystalline shell 204. Thenanocrystalline core 202 has a length axis (a_(CORE)), a width axis(b_(CORE)) and a depth axis (c_(CORE)), the depth axis provided into andout of the plane shown in FIG. 2. Likewise, the nanocrystalline shell204 has a length axis (a_(SHELL)), a width axis (b_(SHELL)) and a depthaxis (c_(SHELL)), the depth axis provided into and out of the planeshown in FIG. 2. The nanocrystalline core 202 has a center 203 and thenanocrystalline shell 204 has a center 205. The nanocrystalline shell204 surrounds the nanocrystalline core 202 in the b-axis direction by anamount 206, as is also depicted in FIG. 2.

The following are attributes of a quantum dot that may be tuned foroptimization, with reference to the parameters provided in FIG. 2, inaccordance with embodiments of the present invention. Nanocrystallinecore 202 diameter (a, b or c) and aspect ratio (e.g., a/b) can becontrolled for rough tuning for emission wavelength (a higher value foreither providing increasingly red emission). A smaller overallnanocrystalline core provides a greater surface to volume ratio. Thewidth of the nanocrystalline shell along 206 may be tuned for yieldoptimization and quantum confinement providing approaches to controlred-shifting and mitigation of surface effects. However, strainconsiderations must be accounted for when optimizing the value ofthickness 206. The length (a_(SHELL)) of the shell is tunable to providelonger radiative decay times as well as increased light absorption. Theoverall aspect ratio of the structure 200 (e.g., the greater ofa_(SHELL)/b_(SHELL) and a_(SHELL)/c_(SHELL)) may be tuned to directlyimpact PLQY. Meanwhile, overall surface/volume ratio for 200 may be keptrelatively smaller to provide lower surface defects, provide higherphotoluminescence, and limit self-absorption. Referring again to FIG. 2,the shell/core interface 208 may be tailored to avoid dislocations andstrain sites. In one such embodiment, a high quality interface isobtained by tailoring one or more of injection temperature and mixingparameters, the use of surfactants, and control of the reactivity ofprecursors, as is described in greater detail below.

In accordance with an embodiment of the present invention, a high PLQYquantum dot is based on a core/shell pairing using an anisotropic core.With reference to FIG. 2, an anisotropic core is a core having one ofthe axes a_(CORE), b_(CORE) or c_(CORE) different from one or both ofthe remaining axes. An aspect ratio of such an anisotropic core isdetermined by the longest of the axes a_(CORE), b_(CORE) or c_(CORE)divided by the shortest of the axes a_(CORE), b_(CORE) or c_(CORE) toprovide a number greater than 1 (an isotropic core has an aspect ratioof 1). It is to be understood that the outer surface of an anisotropiccore may have rounded or curved edges (e.g., as in an ellipsoid) or maybe faceted (e.g., as in a stretched or elongated tetragonal or hexagonalprism) to provide an aspect ratio of greater than 1 (note that a sphere,a tetragonal prism, and a hexagonal prism are all considered to have anaspect ratio of 1 in keeping with embodiments of the present invention).

A workable range of aspect ratio for an anisotropic nanocrystalline corefor a quantum dot may be selected for maximization of PLQY. For example,a core essentially isotropic may not provide advantages for increasingPLQY, while a core with too great an aspect ratio (e.g., 2 or greater)may present challenges synthetically and geometrically when forming asurrounding shell. Furthermore, embedding the core in a shell composedof a material different than the core may also be used enhance PLQY of aresulting quantum dot.

Accordingly, in an embodiment, a semiconductor structure includes ananisotropic nanocrystalline core composed of a first semiconductormaterial and having an aspect ratio between, but not including, 1.0 and2.0. The semiconductor structure also includes a nanocrystalline shellcomposed of a second, different, semiconductor material at leastpartially surrounding the anisotropic nanocrystalline core. In one suchembodiment, the aspect ratio of the anisotropic nanocrystalline core isapproximately in the range of 1.01-1.2 and, in a particular embodiment,is approximately in the range of 1.1-1.2. In the case of rounded edges,then, the nanocrystalline core may be substantially, but not perfectly,spherical. However, the nanocrystalline core may instead be faceted. Inan embodiment, the anisotropic nanocrystalline core is disposed in anasymmetric orientation with respect to the nanocrystalline shell, asdescribed in greater detail in the example below.

Another consideration for maximization of PLQY in a quantum dotstructure is to provide an asymmetric orientation of the core within asurrounding shell. For example, referring again to FIG. 2, the center203 of the core 202 may be misaligned with (e.g., have a differentspatial point than) the center 205 of the shell 204. In an embodiment, asemiconductor structure includes an anisotropic nanocrystalline corecomposed of a first semiconductor material. The semiconductor structurealso includes a nanocrystalline shell composed of a second, different,semiconductor material at least partially surrounding the anisotropicnanocrystalline core. The anisotropic nanocrystalline core is disposedin an asymmetric orientation with respect to the nanocrystalline shell.In one such embodiment, the nanocrystalline shell has a long axis (e.g.,a_(SHELL)), and the anisotropic nanocrystalline core is disposedoff-center along the long axis. In another such embodiment, thenanocrystalline shell has a short axis (e.g., b_(SHELL)), and theanisotropic nanocrystalline core is disposed off-center along the shortaxis. In yet another embodiment, however, the nanocrystalline shell hasa long axis (e.g., a_(SHELL)) and a short axis (e.g., b_(SHELL)), andthe anisotropic nanocrystalline core is disposed off-center along boththe long and short axes.

With reference to the above described nanocrystalline core andnanocrystalline shell pairings, in an embodiment, the nanocrystallineshell completely surrounds the anisotropic nanocrystalline core. In analternative embodiment, however, the nanocrystalline shell onlypartially surrounds the anisotropic nanocrystalline core, exposing aportion of the anisotropic nanocrystalline core, e.g., as in a tetrapodgeometry or arrangement. In an embodiment, the nanocrystalline shell isan anisotropic nanocrystalline shell, such as a nano-rod, that surroundsthe anisotropic nanocrystalline core at an interface between theanisotropic nanocrystalline shell and the anisotropic nanocrystallinecore. The anisotropic nanocrystalline shell passivates or reduces trapstates at the interface. The anisotropic nanocrystalline shell may also,or instead, deactivate trap states at the interface.

With reference again to the above described nanocrystalline core andnanocrystalline shell pairings, in an embodiment, the first and secondsemiconductor materials (core and shell, respectively) are eachmaterials such as, but not limited to, Group II-VI materials, GroupIII-V materials, Group IV-VI materials, Group I-III-VI materials, orGroup II-IV-VI materials and, in one embodiment, are monocrystalline. Inone such embodiment, the first and second semiconductor materials areboth Group II-VI materials, the first semiconductor material is cadmiumselenide (CdSe), and the second semiconductor material is one such as,but not limited to, cadmium sulfide (CdS), zinc sulfide (ZnS), or zincselenide (ZnSe). In an embodiment, the semiconductor structure furtherincludes a nanocrystalline outer shell at least partially surroundingthe nanocrystalline shell and, in one embodiment, the nanocrystallineouter shell completely surrounds the nanocrystalline shell. Thenanocrystalline outer shell is composed of a third semiconductormaterial different from the first and second semiconductor materials. Ina particular such embodiment, the first semiconductor material iscadmium selenide (CdSe), the second semiconductor material is cadmiumsulfide (CdS), and the third semiconductor material is zinc sulfide(ZnS).

With reference again to the above described nanocrystalline core andnanocrystalline shell pairings, in an embodiment, the semiconductorstructure (i.e., the core/shell pairing in total) has an aspect ratioapproximately in the range of 1.5-10 and, 3-6 in a particularembodiment. In an embodiment, the nanocrystalline shell has a long axisand a short axis. The long axis has a length approximately in the rangeof 5-40 nanometers. The short axis has a length approximately in therange of 1-5 nanometers greater than a diameter of the anisotropicnanocrystalline core parallel with the short axis of the nanocrystallineshell. In a specific such embodiment, the anisotropic nanocrystallinecore has a diameter approximately in the range of 2-5 nanometers. Thethickness of the nanocrystalline shell on the anisotropicnanocrystalline core along a short axis of the nanocrystalline shell isapproximately in the range of 1-5 nanometers of the second semiconductormaterial.

With reference again to the above described nanocrystalline core andnanocrystalline shell pairings, in an embodiment, the anisotropicnanocrystalline core and the nanocrystalline shell form a quantum dot.In one such embodiment, the quantum dot has a photoluminescence quantumyield (PLQY) of at least 90%. Emission from the quantum dot may bemostly, or entirely, from the nanocrystalline core. For example, in anembodiment, emission from the anisotropic nanocrystalline core is atleast approximately 75% of the total emission from the quantum dot. Anabsorption spectrum and an emission spectrum of the quantum dot may beessentially non-overlapping. For example, in an embodiment, anabsorbance ratio of the quantum dot based on absorbance at 400nanometers versus absorbance at an exciton peak for the quantum dot isapproximately in the range of 5-35.

In an embodiment, a quantum dot based on the above describednanocrystalline core and nanocrystalline shell pairings is adown-converting quantum dot. However, in an alternative embodiment, thequantum dot is an up-shifting quantum dot. In either case, a lightingapparatus may include a light emitting diode and a plurality of quantumdots such as those described above. The quantum dots may be appliedproximal to the LED and provide down-conversion or up-shifting of lightemitted from the LED. Thus, semiconductor structures according to thepresent invention may be advantageously used in solid state lighting.The visible spectrum includes light of different colors havingwavelengths between about 380 nm and about 780 nm that are visible tothe human eye. An LED will emit a UV or blue light which isdown-converted (or up-shifted) by semiconductor structures describedherein. Any suitable ratio of emission color from the semiconductorstructures may be used in devices of the present invention. LED devicesaccording to embodiments of the present invention may have incorporatedtherein sufficient quantity of semiconductor structures (e.g., quantumdots) described herein capable of down-converting any available bluelight to red, green, yellow, orange, blue, indigo, violet or othercolor. These structures may also be used to downconvert or upconvertlower energy light (green, yellow, etc) from LED devices, as long as theexcitation light produces emission from the structures.

Semiconductor structures according to embodiments of the presentinvention may be advantageously used in biological imaging in, e.g., oneor more of the following environments: fluorescence resonance energytransfer (FRET) analysis, gene technology, fluorescent labeling ofcellular proteins, cell tracking, pathogen and toxin detection, in vivoanimal imaging or tumor biology investigation. Accordingly, embodimentsof the present invention contemplate probes having quantum dotsdescribed herein.

Semiconductor structures according to embodiments of the presentinvention may be advantageously used in photovoltaic cells in layerswhere high PLQY is important. Accordingly, embodiments of the presentinvention contemplate photovoltaic devices using quantum dots describedherein.

Semiconductor structures according to embodiments of the presentinvention may be advantageously used in display applications as phosphorreplacements where high PLQY is important. Accordingly, embodiments ofthe present invention contemplate display devices using quantum dotsdescribed herein.

There are various synthetic approaches for fabricating CdSe quantumdots. For example, in an embodiment, under an inert atmosphere (e.g.,ultra high purity (UHP) argon), cadmium oxide (CdO) is dissociated inthe presence of surfactant (e.g., octadecylphosphonic acid (ODPA)) andsolvent (e.g., trioctylphosphine oxide (TOPO); trioctylphosphine (TOP))at high temperatures (e.g., 350-380 degrees Celsius). Resulting Cd²⁺cations are exposed by rapid injection to solvated selenium anions(Se²⁻), resulting in a nucleation event forming small CdSe seeds. Theseeds continue to grow, feeding off of the remaining Cd²⁺ and Se²⁻available in solution, with the resulting quantum dots being stabilizedby surface interactions with the surfactant in solution (ODPA). Theaspect ratio of the CdSe seeds is typically between 1 and 2, as dictatedby the ratio of the ODPA to the Cd concentration in solution. Thequality and final size of these cores is affected by several variablessuch as, but not limited to, reaction time, temperature, reagentconcentration, surfactant concentration, moisture content in thereaction, or mixing rate. The reaction is targeted for a narrow sizedistribution of CdSe seeds (assessed by transmission electron microscopy(TEM)), typically a slightly cylindrical seed shape (also assessed byTEM) and CdSe seeds exhibiting solution stability over time (assessed byPLQY and scattering in solution).

For the cadmium sulfide (CdS) shell growth on the CdSe seeds, ornanocrystalline cores, under an inert atmosphere (e.g. UHP argon),cadmium oxide (CdO) is dissociated in the presence of surfactants (e.g.,ODPA and hexylphosphonic acid (HPA)) and solvent (e.g. TOPO and/or TOP)at high temperatures (e.g., 350-380 degrees Celsius). The resulting Cd²⁺cations in solution are exposed by rapid injection to solvated sulfuranions (S²⁻) and CdSe cores. Immediate growth of the CdS shell aroundthe CdSe core occurs. The use of both a short chain and long chainphosphonic acid promotes enhanced growth rate at along the c-axis of thestructure, and slower growth along the a-axis, resulting in a rod-shapedcore/shell nanomaterial.

CdSe/CdS core-shell quantum dots have been shown in the literature toexhibit respectable quantum yields (e.g., 70-75%). However, thepersistence of surface trap states (which decrease overallphotoluminescent quantum yield) in these systems arises from a varietyof factors such as, but not limited to, strain at the core-shellinterface, high aspect ratios (ratio of rod length to rod width of thecore/shell pairing) which lead to larger quantum dot surface arearequiring passivation, or poor surface stabilization of the shell.

In order to address the above synthetic limitations on the quality ofquantum dots formed under conventional synthetic procedures, in anembodiment, a multi-faceted approach is used to mitigate or eliminatesources of surface trap states in quantum dot materials. For example,lower reaction temperatures during the core/shell pairing growth yieldsslower growth at the CdSe—CdS interface, giving each material sufficienttime to orient into the lowest-strain positions. Aspect ratios arecontrolled by changing the relative ratios of surfactants in solution aswell as by controlling temperature. Increasing an ODPA/HPA ratio inreaction slows the rapid growth at the ends of the core/shell pairingsby replacing the facile HPA surfactant with the more obstructive ODPAsurfactant. In addition, lowered reaction temperatures are also used tocontribute to slowed growth at the ends of the core/shell pairings. Bycontrolling these variables, the aspect ratio of the core/shell pairingis optimized for quantum yield. In one such embodiment, followingdetermination of optimal surfactant ratios, overall surfactantconcentrations are adjusted to locate a PLQY maximum while maintaininglong-term stability of the fabricated quantum dots in solution.Furthermore, in an embodiment, aspect ratios of the seed or core (e.g.,as opposed to the seed/shell pairing) are limited to a range between,but not including 1.0 and 2.0 in order to provide an appropriategeometry for high quality shell growth thereon.

In another aspect, an additional or alternative strategy for improvingthe interface between CdSe and CdS includes, in an embodiment,chemically treating the surface of the CdSe cores prior to reaction.CdSe cores are stabilized by long chain surfactants (ODPA) prior tointroduction into the CdS growth conditions. Reactive ligand exchangecan be used to replace the ODPA surfactants with ligands which areeasier to remove (e.g., primary or secondary amines), facilitatingimproved reaction between the CdSe core and the CdS growth reagents.

In addition to the above factors affecting PLQY in solution,self-absorption may negatively affect PLQY when these materials are castinto films. This phenomenon may occur when CdSe cores re-absorb lightemitted by other quantum dots. In one embodiment, the thickness of theCdS shells around the same CdSe cores is increased in order to increasethe amount of light absorbed per core/shell pairing, while keeping theparticle concentration the same or lower in films including the quantumdot structures. The addition of more Cd and S to the shell formationreaction leads to more shell growth, while an optimal surfactant ratioallows targeting of a desired aspect ratio and solubility of thecore/shell pairing.

Accordingly, in an embodiment, an overall method of fabricating asemiconductor structure, such as the above described quantum dotstructures, includes forming an anisotropic nanocrystalline core from afirst semiconductor material. A nanocrystalline shell is formed from asecond, different, semiconductor material to at least partially surroundthe anisotropic nanocrystalline core. In one such embodiment, theanisotropic nanocrystalline core has an aspect ratio between, but notincluding, 1.0 and 2.0, as described above.

With reference to the above described general method for fabricating ananocrystalline core and nanocrystalline shell pairing, in anembodiment, prior to forming the nanocrystalline shell, the anisotropicnanocrystalline core is stabilized in solution with a surfactant. In onesuch embodiment, the surfactant is octadecylphosphonic acid (ODPA). Inanother such embodiment, the surfactant acts as a ligand for theanisotropic nanocrystalline core. In that embodiment, the method furtherincludes, prior to forming the nanocrystalline shell, replacing thesurfactant ligand with a second ligand, the second ligand more labilethan the surfactant ligand. In a specific such embodiment, the secondligand is one such as, but not limited to, a primary amine or asecondary amine.

With reference again to the above described general method forfabricating a nanocrystalline core and nanocrystalline shell pairing, inan embodiment, forming the nanocrystalline shell includes forming thesecond semiconductor material in the presence of a mixture ofsurfactants. In one such embodiment, the mixture of surfactants includesa mixture of octadecylphosphonic acid (ODPA) and hexylphosphonic acid(HPA). In a specific such embodiment, forming the nanocrystalline shellincludes tuning the aspect ratio of the nanocrystalline shell by tuningthe ratio of ODPA versus HPA. Forming the second semiconductor materialin the presence of the mixture of surfactants may also, or instead,include using a solvent such as, but not limited to, trioctylphosphineoxide (TOPO) and trioctylphosphine (TOP).

With reference again to the above described general method forfabricating a nanocrystalline core and nanocrystalline shell pairing, inan embodiment, forming the anisotropic nanocrystalline core includesforming at a temperature approximately in the range of 350-380 degreesCelsius. In an embodiment, forming the anisotropic nanocrystalline coreincludes forming a cadmium selenide (CdSe) nanocrystal from cadmiumoxide (CdO) and selenium (Se) in the presence of a surfactant at atemperature approximately in the range of 300-400 degrees Celsius. Thereaction is arrested prior to completion. In one such embodiment,forming the nanocrystalline shell includes forming a cadmium sulfide(CdS) nanocrystalline layer on the CdSe nanocrystal from cadmium oxide(CdO) and sulfur (S) at a temperature approximately in the range of120-380 degrees Celsius. That reaction is also arrested prior tocompletion.

The aspect ratio of the fabricated semiconductor structures may becontrolled by one of several methods. For example, ligand exchange maybe used to change the surfactants and/or ligands and alter the growthkinetics of the shell and thus the aspect ratio. Changing the coreconcentration during core/shell growth may also be exploited. Anincrease in core concentration and/or decrease concentration ofsurfactants results in lower aspect ratio core/shell pairings.Increasing the concentration of a shell material such as S for CdS willincrease the rate of growth on the ends of the core/shell pairings,leading to longer, higher aspect ratio core/shell pairings.

As mentioned above, in one embodiment of the present invention,nanocrystalline cores undergo a reactive ligand exchange which replacescore surfactants with ligands that are easier to remove (e.g., primaryor secondary amines), facilitating better reaction between the CdSe coreand the CdS growth reagents. In one embodiment, cores used herein haveligands bound or associated therewith. Attachment may be by dativebonding, Van der Waals forces, covalent bonding, ionic bonding or otherforce or bond, and combinations thereof. Ligands used with the cores mayinclude one or more functional groups to bind to the surface of thenanocrystals. In a specific such embodiment, the ligands have afunctional group with an affinity for a hydrophobic solvent.

In an embodiment, lower reaction temperatures during shell growth yieldsslower growth at the core/shell interface. While not wishing to be boundby any particular theory or principle it is believed that this methodallows both core and shell seed crystals time to orient into theirlowest-strain positions during growth. Growth at the ends of thecore/shell pairing structure is facile and is primarily governed by theconcentration of available precursors (e.g., for a shell of CdS this isCd, S:TOP). Growth at the sides of the core/shell pairings is morestrongly affected by the stabilizing ligands on the surface of thecore/shell pairing. Ligands may exist in equilibrium between thereaction solution and the surface of the core/shell pairing structure.Lower reaction temperatures may tilt this equilibrium towards moreligands being on the surface, rendering it more difficult for growthprecursors to access this surface. Hence, growth in the width directionis hindered by lower temperature, leading to higher aspect ratiocore/shell pairings.

In general consideration of the above described semiconductor or quantumdot structures and methods of fabricating such semiconductor or quantumdot structures, in an embodiment, quantum dots are fabricated to have anabsorbance in the blue or ultra-violet (V) regime, with an emission inthe visible (e.g., red, orange, yellow, green, blue, indigo and violet,but particularly red and green) regime. The above described quantum dotsmay advantageously have a high PLQY with limited self-absorption,possess a narrow size distribution for cores, provide core stabilityover time (e.g., as assessed by PLQY and scattering in solution), andexhibit no major product loss during purification steps. Quantum dotsfabricated according one or more of the above embodiments may have adecoupled absorption and emission regime, where the absorption iscontrolled by the shell and the emission is controlled by the core. Inone such embodiment, a dimension of the core correlates with emissioncolor, e.g., a core diameter progressing from 3-5.5 nanometerscorrelates approximately to a green→yellow→red emission progression.

With reference to the above described embodiments concerningsemiconductor structures, such as quantum dots, and methods offabricating such structures, the concept of a crystal defect, ormitigation thereof, may be implicated. For example, a crystal defect mayform in, or be precluded from forming in, a nanocrystalline core or in ananocrystalline shell, at an interface of the core/shell pairing, or atthe surface of the core or shell. In an embodiment, a crystal defect isa departure from crystal symmetry caused by on or more of free surfaces,disorder, impurities, vacancies and interstitials, dislocations, latticevibrations, or grain boundaries. Such a departure may be referred to asa structural defect or lattice defect. Reference to an exciton is to amobile concentration of energy in a crystal formed by an excitedelectron and an associated hole. An exciton peak is defined as the peakin an absorption spectrum correlating to the minimum energy for a groundstate electron to cross the band gap. The core/shell quantum dotabsorption spectrum appears as a series of overlapping peaks that getlarger at shorter wavelengths. Because of their discrete electron energylevels, each peak corresponds to an energy transition between discreteelectron-hole (exciton) energy levels. The quantum dots do not absorblight that has a wavelength longer than that of the first exciton peak,also referred to as the absorption onset. The wavelength of the firstexciton peak, and all subsequent peaks, is a function of the compositionand size of the quantum dot. An absorbance ratio is absorbance of thecore/shell nanocrystal at 400 nm divided by the absorbance of thecore/shell nanocrystal at the first exciton peak. Photoluminescencequantum yield (PLQY) is defined as the ratio of the number of photonsemitted to the number of photons absorbed. Core/shell pairing describedherein may have a Type 1 band alignment, e.g., the core band gap isnested within the band gap of the shell. Emission wavelength may bedetermined by controlling the size and shape of the core nanocrystal,which controls the band gap of the core. Emission wavelength may also beengineered by controlling the size and shape of the shell. In anembodiment, the amount/volume of shell material is much greater thanthat of the core material. Consequently, the absorption onset wavelengthis mainly controlled by the shell band gap. Core/shell quantum dots inaccordance with an embodiment of the present invention have anelectron-hole pair generated in the shell which is then funneled intothe core, resulting in recombination and emission from the core quantumdot. Preferably emission is substantially from the core of the quantumdot.

Measurement of Photoluminescence Quantum Yield (PLQY) may be performedaccording to the method disclosed in Laurent Pones et al. “AbsoluteMeasurements of Photoluminescence Quantum Yields of Solutions Using anIntegrating Sphere”, Journal of Fluorescence (2006) DOI:10.1007/s10895-005-0054-8, Springer Science+Business Media, Inc. As anexample, FIG. 3 illustrates a schematic of an integrating sphere 300 formeasuring absolute photoluminescence quantum yield, in accordance withan embodiment of the present invention. The integrating sphere 300includes a sample holder 302, a spectrometer 304, a calibrated lightsource 306 and an ultra-violet (UV) LED 308. FIG. 4 is a plot 400 ofphoton counts as a function of wavelength in nanometers for sample andreference emission spectra used in the measurement of photoluminescencequantum yield, in accordance with an embodiment of the presentinvention. Referring to plot 400, both excitation and emission peaks fora sample are calibrated against corresponding excitation and emissionpeaks for a reference.

In an embodiment, PLQY is measured with a Labsphere™ 6″ integratingsphere, a Labsphere™ LPS-100-0105 calibrated white light source, a 3.8W, 405 nm Thorlabs™ M405L2 UV LED and an Ocean Optics™ USB4000-VIS-NIRspectrometer. The spectrometer and UV LED are coupled into the sphereusing Ocean Optics™ UV-Vis optical fibers. The spectrometer fiber isattached to a lens in a port at the side of the sphere at 90 degreesrelative to the excitation source. The lens is behind a flat baffle toensure only diffuse light reaches the lens. The calibrated white lightsource is affixed to a port in the side of the sphere, at 90° to boththe excitation source and the spectrometer port. Custom made sampleholders are used to hold solid and solution (cuvette) samples and torotate samples between direct and indirect measurement positions. Sampleholders are coated with a barium sulfate diffuse reflective material.Before measurements are recorded, the calibrated white light source isused to calibrate the spectrometer as a function of wavelength(translating counts per second into relative intensity vs. wavelength).To measure PLQY, a reference sample is inserted into the sphere, and theexcitation source LED signal is recorded. This reference sample isgenerally a blank, such as a cuvette containing a solvent or a samplewithout quantum dots, so as to only measure the properties of thequantum dots. If it is desirable to measure the properties of thematrix, the blank may be only the substrate. The sample is then insertedinto the sphere, in direct beam line for direct measurements, and out ofthe beam for indirect measurements. The spectrum is recorded and splitinto excitation and emission bands, each is integrated, and the numberof photons emitted per photons absorbed is the photoluminescence quantumyield (PLQY), which is equal to the difference between sample emissionand reference emission divided by the difference of reference excitationand sample excitation.

Quantum dots according to embodiments of the present invention have aPLQY between 90-100%, or at least 90%, more preferably at least 91%,more preferably at least 92%, more preferably at least 93%, morepreferably at least 94%, more preferably at least 95%, more preferablyat least 96%, more preferably at least 97%, more preferably at least98%, more preferably at least 99% and most preferably 100%. FIG. 5 is aplot 500 including a UV-Vis absorbance spectrum 502 and photoluminescentemission spectrum 504 for red CdSe/CdS core/shell quantum dots, inaccordance with an embodiment of the present invention. The quantum dotshave essentially no overlapping absorption and emission bands and havingan absorbance ratio of about 24. The PLQY was determined to be 94% at617 nm. The average length (from transmission electron microscopy (TEM)data) is 27 nm±3.3 nm. The average width (from TEM data) is 7.9 nm±1.1nm. The average aspect ratio (from TEM data) is 3.5±0.6. FIG. 6 is aplot 600 including a UV-Vis absorbance spectrum 602 and photoluminescentemission spectrum 604 for a green CdSe/CdS core/shell quantum dot, inaccordance with an embodiment of the present invention. The quantum dothas a small extent of overlapping absorption and emission bands and hasan absorbance ratio of 16 (plus or minus one).

In another aspect, semiconductor structures having a nanocrystallinecore and corresponding nanocrystalline shell and insulator coating aredescribed. Particularly, coated quantum dots structures and methods ofmaking such structures are described below. In an embodiment, core/shellquantum dots are coated with silica by a method resulting incompositions having photoluminescence quantum yields between 90 and100%. In one such embodiment, semiconductor structures are coated withsilica using a reverse micelle method. A quantum dot may be engineeredso that emission is substantially from the core.

Prior art quantum dots may have poor nanocrystal surface and crystallinequality as a result of prior art synthetic techniques not being capableof treating the nanocrystal surface in ways capable of achieving PLQYsabove 90 percent. For example, the surface of a nanocrystallinecore/shell pairing may have a large number of dangling bonds which actas trap states reducing emission and, therefore, PLQY. Prior arttechniques to modify the quantum dot surface include coating quantumdots with silica. However, prior art silica coated quantum dots do notachieve the PLQY necessary for continued use in solid state lightingdevices.

In conventional approaches, silica coatings can encapsulate more thanone particle (e.g., quantum dot structure) at a time, or the approacheshave resulted in incomplete encapsulation. One such conventionalapproach included coating a quantum dot with silica using self-assembledmicelles. The approach requires the presence of a majority of a polarsolvent to form a micelle. The requirement is for polar solventenvironments to generate the encapsulating micelle, and thus limits thetechnique to aqueous based applications, such as biological tagging andimaging. Quantum dots with a hydrophobic surfactant or ligand attachedare aqueous solution insoluble and thus silica cannot be precipitatedwith the nanocrystals within the aqueous domains of the micro emulsion.Ligand exchange reactions may be required which then leads to surfacequality degradation. However, conventional quantum dot systems oftenrely on the weak dative Van der Waals bonding of ligands such asphosphonic acids, amines, and carboxylic acids to maintain thestructures in solution and protect and passivate the surface of thequantum dot.

The integration of a quantum dot into a product may require protectionfor chemical compatibility with the solution environment duringprocessing, and ultimately the plastic or gel used for encapsulation.Without such compatibility, particles are likely to aggregate and/orredistribute themselves within the matrix, an unacceptable occurrencein, for example, a solid state lighting product. Protection of thesurface and maintenance of an electronically uniform environment alsoensures that the density of non-radiative pathways (traps) is minimized,and that the emission energy (color) is as uniform as possible.Furthermore, the surface is protected from further chemical reactionwith environmental degradants such as oxygen. This is particularlyimportant for LED applications, where the quantum dot must toleratetemperatures as high as 200 degrees Celsius and constant high-intensityillumination with high-energy light. However, the weak surface bondingof prior art quantum dot ligands are non-ideal for the processing andlong-term performance required of an LED product, as they allowdegradants access to the quantum dot surface.

In accordance with an embodiment of the present invention, core/shellquantum dots coated with silica and other ligands to provide a structurehaving a high PLQY. One embodiment exploits a sol-gel process whichencapsulates each quantum dot individually in a silica shell, resultingin a very stable high PLQY quantum dot particle. The coated quantum dotsdisclosed herein may advantageously possess a narrow size distributionfor CdSe core stability over time (assessed by PLQY and scattering insolution).

In a general embodiment, a semiconductor structure includes ananocrystalline core composed of a first semiconductor material. Thesemiconductor structure also includes a nanocrystalline shell composedof a second, different, semiconductor material at least partiallysurrounding the nanocrystalline core. An insulator layer encapsulates,e.g., coats, the nanocrystalline shell and nanocrystalline core. Thus,coated semiconductor structures include coated structures such as thequantum dots described above. For example, in an embodiment, thenanocrystalline core is anisotropic, e.g., having an aspect ratiobetween, but not including, 1.0 and 2.0. In another example, in anembodiment, the nanocrystalline core is anisotropic and isasymmetrically oriented within the nanocrystalline shell. In anembodiment, the nanocrystalline core and the nanocrystalline shell forma quantum dot.

With reference to the above described coated nanocrystalline core andnanocrystalline shell pairings, in an embodiment, the insulator layer isbonded directly to the nanocrystalline shell. In one such embodiment,the insulator layer passivates an outermost surface of thenanocrystalline shell. In another embodiment, the insulator layerprovides a barrier for the nanocrystalline shell and nanocrystallinecore impermeable to an environment outside of the insulator layer. Inany case, the insulator layer may encapsulate only a singlenanocrystalline shell/nanocrystalline core pairing. In an embodiment,the semiconductor structure further includes a nanocrystalline outershell at least partially surrounding the nanocrystalline shell, betweenthe nanocrystalline shell and the insulator layer. The nanocrystallineouter shell is composed of a third semiconductor material different fromthe semiconductor material of the shell and, possibly, different fromthe semiconductor material of the core.

With reference again to the above described coated nanocrystalline coreand nanocrystalline shell pairings, in an embodiment, the insulatorlayer is composed of a layer of material such as, but not limited to,silica (SiO_(x)), titanium oxide (TiO_(x)), zirconium oxide (ZrO_(x)),alumina (AlO_(x)), or hafnia (HfO_(x)). In one such embodiment, thelayer is a layer of silica having a thickness approximately in the rangeof 3-30 nanometers. In an embodiment, the insulator layer is anamorphous layer.

With reference again to the above described coated nanocrystalline coreand nanocrystalline shell pairings, in an embodiment, an outer surfaceof the insulator layer is ligand-free. However, in an alternativeembodiment, an outer surface of the insulator layer isligand-functionalized. In one such embodiment, the outer surface of theinsulator layer is ligand-functionalized with a ligand such as, but notlimited to, a silane having one or more hydrolyzable groups or afunctional or non-functional bipodal silane. In another such embodiment,the outer surface of the insulator layer is ligand-functionalized with aligand such as, but not limited to, mono-, di-, or tri-alkoxysilaneswith three, two or one inert or organofunctional substituents of thegeneral formula (R¹O)₃SiR²; (R¹O)₂SiR²R³; (R¹O) SiR²R³R⁴, where R¹ ismethyl, ethyl, propyl, isopropyl, or butyl, R², R³ and R⁴ are identicalor different and are H substituents, alkyls, alkenes, alkynes, aryls,halogeno-derivates, alcohols, (mono, di, tri, poly) ethyleneglycols,(secondary, tertiary, quaternary) amines, diamines, polyamines, azides,isocyanates, acrylates, metacrylates, epoxies, ethers, aldehydes,carboxylates, esters, anhydrides, phosphates, phosphines, mercaptos,thiols, sulfonates, and are linear or cyclic, a silane with the generalstructure (R¹O)₃Si—(CH₂)_(n)—R—(CH₂)_(n)—Si(RO)₃ where R and R¹ is H oran organic substituent selected from the group consisting of alkyls,alkenes, alkynes, aryls, halogeno-derivates, alcohols, (mono, di, tri,poly) ethyleneglycols, (secondary, tertiary, quaternary) amines,diamines, polyamines, azides, isocyanates, acrylates, metacrylates,epoxies, ethers, aldehydes, carboxylates, esters, anhydrides,phosphates, phosphines, mercaptos, thiols, sulfonates, and are linear orcyclic, a chlorosilane, or an azasilane. In another such embodiment, theouter surface of the insulator layer is ligand-functionalized with aligand such as, but not limited to, organic or inorganic compounds withfunctionality for bonding to a silica surface by chemical ornon-chemical interactions such as but not limited to covalent, ionic,H-bonding, or Van der Waals forces. In yet another such embodiment, theouter surface of the insulator layer is ligand-functionalized with aligand such as, but not limited to, the methoxy and ethoxy silanes(MeO)₃SiAllyl, (MeO)₃SiVinyl, (MeO)₂SiMeVinyl, (EtO)₃SiVinyl,EtOSi(Vinyl)₃, mono-methoxy silanes, chloro-silanes, or1,2-bis-(triethoxysilyl)ethane. In any case, in an embodiment, the outersurface of the insulator layer is ligand-functionalized to impartsolubility, dispersability, heat stability, photo-stability, or acombination thereof, to the semiconductor structure. For example, in oneembodiment, the outer surface of the insulator layer includes OH groupssuitable for reaction with an intermediate linker to link smallmolecules, oligomers, polymers or macromolecules to the outer surface ofthe insulator layer, the intermediate linker one such as, but notlimited to, an epoxide, a carbonyldiimidazole, a cyanuric chloride, oran isocyanate.

With reference again to the above described coated nanocrystalline coreand nanocrystalline shell pairings, in an embodiment, thenanocrystalline core has a diameter approximately in the range of 2-5nanometers. The nanocrystalline shell has a long axis and a short axis,the long axis having a length approximately in the range of 5-40nanometers, and the short axis having a length approximately in therange of 1-5 nanometers greater than the diameter of the nanocrystallinecore. The insulator layer has a thickness approximately in the range of1-20 nanometers along an axis co-axial with the long axis and has athickness approximately in the range of 3-30 nanometers along an axisco-axial with the short axis.

A lighting apparatus may include a light emitting diode and a pluralityof semiconductor structures which, e.g., act to down convert lightabsorbed from the light emitting diode. For example, in one embodiment,each semiconductor structure includes a quantum dot having ananocrystalline core composed of a first semiconductor material and ananocrystalline shell composed of a second, different, semiconductormaterial at least partially surrounding the nanocrystalline core. Eachquantum dot has a photoluminescence quantum yield (PLQY) of at least90%. An insulator layer encapsulates each quantum dot.

As described briefly above, an insulator layer may be formed toencapsulate a nanocrystalline shell and anisotropic nanocrystallinecore. For example, in an embodiment, a layer of silica is formed using areverse micelle sol-gel reaction. In one such embodiment, using thereverse micelle sol-gel reaction includes dissolving the nanocrystallineshell/nanocrystalline core pairing in a first non-polar solvent to forma first solution. Subsequently, the first solution is added along with aspecies such as, but not limited to, 3-aminopropyltrimethoxysilane(APTMS), 3-mercapto-trimethoxysilane, or a silane comprising aphosphonic acid or carboxylic acid functional group, to a secondsolution having a surfactant dissolved in a second non-polar solvent.Subsequently, ammonium hydroxide and tetraorthosilicate (TEOS) are addedto the second solution.

Thus, semiconductor nanocrystals coated with silica according to thepresent invention may be made by a sol-gel reaction such as a reversemicelle method. As an example, FIG. 7 illustrates operations in areverse micelle approach to coating a semiconductor structure, inaccordance with an embodiment of the present invention. Referring topart A of FIG. 7, a quantum dot heterostructure (QDH) 702 (e.g., ananocrystalline core/shell pairing) has attached thereto a plurality ofTOPO ligands 704 and TOP ligands 706. Referring to part B, the pluralityof TOPO ligands 704 and TOP ligands 706 are exchanged with a pluralityof Si(OCH₃)₃(CH₂)₃NH₂ ligands 708. The structure of part B is thenreacted with TEOS (Si(OEt)₄) and ammonium hydroxide (NH₄OH) to form asilica coating 710 surrounding the QDH 702, as depicted in part C ofFIG. 7. FIG. 8 is a transmission electron microscope (TEM) image 800 ofsilica coated 802 CdSe/CdS core/shell quantum dots 804 having completesilica encapsulation, in accordance with an embodiment of the presentinvention. Thus, a reverse micelle is formed after adding ammoniumhydroxide and tetraethylorthosilicate (TEOS), the source for the silicacoating. TEOS diffuses through the micelle and is hydrolyzed by ammoniato form a uniform SiO₂ shell on the surface of the quantum dot. Thisapproach may offer great flexibility to incorporate quantum dots ofdifferent sizes. In one such embodiment, the thickness of the insulatorlayer formed depends on the amount of TEOS added to the second solution.

With reference again to the above described method of forming coatednanocrystalline core and nanocrystalline shell pairings, in anembodiment, the first and second non-polar solvents are cyclohexane. Inan embodiment, forming the coating layer includes forming a layer ofsilica and further includes using a combination of dioctyl sodiumsulfosuccinate (AOT) and tetraorthosilicate (TEOS). In anotherembodiment, however, forming the layer includes forming a layer ofsilica and further includes using a combination of polyoxyethylene (5)nonylphenylether and tetraorthosilicate (TEOS). In another embodiment,however, forming the layer includes forming a layer of silica andfurther includes using cationic surfactants such as CTAB(cetyltrimethylammonium bromide), anionic surfactants, non-ionicsurfactants, or pluronic surfactants such as Pluronic F 127 (an ethyleneoxide/propylene oxide block co-polymer) as well as mixtures ofsurfactants.

Upon initiation of growth of a silica shell, the final size of thatshell may be directly related to the amount of TEOS in the reactionsolution. Silica coatings according to embodiments of the presentinvention may be conformal to the core/shell QDH or non-conformal. Asilica coating may be between about 3 nm and 30 nm thick. The silicacoating thickness along the c-axis may be as small as about 1 nm or aslarge as about 20 nm. The silica coating thickness along the a-axis maybe between about 3 nm and 30 nm. Once silica shelling is complete, theproduct is washed with solvent to remove any remaining ligands. Thesilica coated quantum dots can then be incorporated into a polymermatrix or undergo further surface functionalization. However, silicashells according to embodiments of the present invention may also befunctionalized with ligands to impart solubility, dispersability, heatstability and photo-stability in the matrix.

In another aspect, quantum dot composite compositions are described. Forexample, the quantum dots (including coated quantum dots) describedabove may be embedded in a matrix material to make a composite using aplastic or other material as the matrix. In an embodiment, compositecompositions including matrix materials and silica coated core/shellquantum dots having photoluminescence quantum yields between 90 and 100%are formed. Such quantum dots may be incorporated into a matrix materialsuitable for down converting in LED applications.

Composites formed by conventional approaches typically suffer fromnon-uniform dispersion of quantum dots throughout the matrix materialwhich can result in particle agglomeration. Agglomeration may be sosevere as to result in emission quenching reducing light output. Anotherproblem is lack of compatibility between the quantum dots and the matrixreduces composite performance. Lack of materials compatibility mayintroduce a discontinuity at the polymer/quantum dot interface wherecomposite failure may initiate when it is deployed in ordinary use.

Accordingly, there remains a need for a composite material having aquantum dot composition in a matrix that is strong, resistant to thermaldegradation, resistant to chemical degradation, provides good adhesionbetween the coated quantum dot and coupling agent and provides goodadhesion between the coupling agent and the polymer matrix. Embodimentsdescribed below include quantum dots incorporated into compositematrixes to produce high refractive index films having a high PLQYsuitable for solid state device lighting including light emittingdiodes.

In an embodiment, an approach for incorporating quantum dots into matrixmaterials includes coating the quantum dot with a silica shell andreacting the silica shell with a silane coupling agent having tworeactive functionalities under the proper conditions. Such anarrangement drives a condensation reaction, binding one end of thesilane to the silica surface and leaving the other end of the moleculeexposed for integration into a matrix. Other approaches include using acurable material such as metal oxide nanocrystals in a matrix material.In the curable material, metal oxide nanocrystals are linked to apolymer matrix via titanate or a zirconate coupling agents as well as asilane coupling agent, where the metal atoms of the coupling agent linkto the oxygen atoms of the metal oxide nanocrystals. Since metal oxidesgenerally do not have a higher refractive index, the curable materialincorporating the metal oxide nanocrystals typically can not achieve arefractive index sufficient to improve the light extraction efficiencyof photons emitted by an LED in a solid-state device. A high refractiveindex material including zinc sulfide (ZnS) in a matrix material isanother approach attempted. In making the high refractive indexmaterial, ZnS colloids are synthesized with ligands having hydroxylfunctional groups that are linked to isocyanate function groups presenton an oligomer backbone in the matrix material.

In a general embodiment, a composite includes a matrix material. Aplurality of semiconductor structures (e.g., quantum dot structureshaving a coated or non-coated core/shell pairing, such as the structuresdescribed above) is embedded in the matrix material. In an embodiment, alighting apparatus includes a light emitting diode and a compositecoating the light emitting diode. The composite may be formed byembedding quantum dots in a matrix material described below.

With reference to the above described composite, in an embodiment, eachof the plurality of semiconductor structures is cross-linked with,polarity bound by, or tethered to the matrix material. In an embodiment,each of the plurality of semiconductor structures is bound to the matrixmaterial by a covalent, dative, or ionic bond. By way of example, FIGS.9A-9C illustrate schematic representations of possible compositecompositions for quantum dot integration, in accordance with anembodiment of the present invention. Referring to FIG. 9A, ananocrystalline core 902A and shell 904A pairing is incorporated into apolymer matrix 906A by active cross-linking through multiple andinterchain binding to form a cross-linked composition 908A. Referring toFIG. 9B, a nanocrystalline core 902B and shell 904B pairing isincorporated into a polymer matrix 906B by polarity-based chemicalsimilarity and dissolution to form a polarity based composition 908B.Referring to FIG. 9C, a nanocrystalline core 902C and shell 904C pairingis incorporated into a polymer matrix 906C by reactive tethering bysparse binding and chemical similarity to form a reactive tetheringbased composition 908C.

With reference again to the above described composite, in an embodiment,one or more of the semiconductor structures further includes a couplingagent covalently bonded to an outer surface of the insulator layer. Forexample, in one such embodiment, the insulator layer includes or is alayer of silica (SiO_(x)), and the coupling agent is a silane couplingagent, e.g., having the formula X_(n)SiY_(4-n), where X is a functionalgroup capable of bonding with the matrix material and is one such as,but not limited to, hydroxyl, alkoxy, isocyanate, carboxyl, epoxy,amine, urea, vinyl, amide, aminoplast and silane, Y is a functionalgroup such as, but not limited to, hydroxyl, phenoxy, alkoxy, hydroxyl,ether, silane or aminoplast, and n is 1, 2 or 3. In another embodiment,however, the coupling agent is one such as, but not limited to, atitanate coupling agent or a zirconate coupling agent. It is to beunderstood that the terms capping agent, capping ligand, ligand andcoupling agent may be used interchangeably as described above and,generally, may include an atom, molecule or other chemical entity ormoiety attached to or capable of being attached to a nanoparticle.Attachment may be by dative bonding, covalent bonding, ionic bonding,Van der Waals forces or other force or bond.

In the case that a silica surface of a silica coated quantum dot ismodified using silane coupling agents having multiple functionalmoieties, coupling to the surface of the silica shell and coupling to amatrix material and/or other matrix additives may be enabled. Such anapproach provides uniform dispersion throughout the composite matrixusing as little effort (e.g., reaction energy) as possible. Strongerphysical and/or chemical bonding between the silica coated quantum dotsand the matrix resin occurs. Also, the silane coupling composition mustbe compatible with both the silica coated quantum dot, which isinorganic, and the polymer matrix, which may be organic. Without beingbound by any particular theory or principle, it is believed that thesilane coupling agent forms a bridge between the silica and the matrixresin when reactive functional groups on the silane coupling agentinteract with functional groups on the surface of the silica and/or thematrix resin. Because the functional groups involved are typically polarin nature, the coupling agent tends to be hydrophilic and readilydispersed in an aqueous size composition.

Matrix materials suitable for embodiments of the present invention maysatisfy the following criteria: they may be optically clear havingtransmission in the 400-700 nm range of greater than 90%, as measured ina UV-Vis spectrometer. They may have a high refractive index betweenabout 1.0 and 2.0, preferably above 1.4 in the 400-700 nm range. Theymay have good adhesion to an LED surface if required and/or aresufficiently rigid for self-supporting applications. They may able tomaintain their properties over a large temperature range, for example−40° C. to 150° C. and over a long period of time (over 50,000 hours ata light intensity typically 1-10 w/cm2 of 450 nm blue light).

Thus, with reference again to the above described composite, in anembodiment, the insulator layer is composed of a layer of silica(SiO_(x)), and the matrix material is composed of a siloxane copolymer.In another embodiment, the matrix material has a UV-Vis spectroscopytransmission of greater than 90% for light in the range of 400-700nanometers. In an embodiment, the matrix material has a refractive indexapproximately in the range of 1-2 for light in the range of 400-700nanometers. In an embodiment, the matrix material is thermally stable ina temperature range of −40-250 degrees Celsius. In an embodiment, thematrix material is composed of a polymer such as, but not limited to,polypropylene, polyethylene, polyesters, polyacetals, polyamides,polyacrylamides, polyimides, polyethers, polyvinylethers, polystyrenes,polyoxides, polycarbonates, polysiloxanes, polysulfones, polyanhydrides,polyamines, epoxies, polyacrylics, polyvinylesters, polyurethane, maleicresins, urea resins, melamine resins, phenol resins, furan resins,polymer blends, polymer alloys, or mixtures thereof. In one suchembodiment, the matrix material is composed of a polysiloxane such as,but not limited to, polydimethylsiloxane (PDMS),polymethylphenylsiloxane, polydiphenylsiloxane and polydiethylsiloxane.In an embodiment, the matrix material is composed of a siloxane such as,but not limited to, dimethylsiloxane or methylhydrogen siloxane.

Additionally, with reference again to the above described composite, inan embodiment, the plurality of semiconductor structures is embeddedhomogeneously in the matrix material. In an embodiment, the compositefurther includes a compounding agent embedded in the matrix material.The compounding agent is one such as, but not limited to, anantioxidant, a pigment, a dye, an antistatic agent, a filler, a flameretardant, an ultra-violet (UV) stabilizer, or an impact modifier. Inanother embodiment, the composite further includes a catalyst embeddedin the matrix material, the catalyst one such as, but not limited to, athiol catalyst or a platinum (Pt) catalyst.

Accordingly, in an embodiment, a method of fabrication includes forminga plurality of semiconductor structures embedded in a matrix material(or embedding preformed semiconductor structures in a matrix material).In one such embodiment, embedding the plurality of semiconductorstructures in the matrix material includes cross-linking, reactivetethering, or ionic bonding the plurality of semiconductor structureswith the matrix material. In an embodiment, the method further includessurface-functionalizing an insulator layer for the semiconductorstructures prior to embedding the plurality of semiconductor structuresin the matrix material. In one such embodiment, thesurface-functionalizing includes treating the insulator layer with asilane coupling agent. However, in an alternative embodiment, coatedsemiconductor structures are embedded in a matrix by using a ligand-freeinsulator layer.

In another embodiment, simple substitution at the surface of the silicacoated quantum dots is effective for stable integration withoutundesired additional viscosity and is suitable to produce alow-viscosity product such as a silicone gel. In one embodiment of thepresent invention a composite incorporates quantum dots which crosslinkwith the matrix through silane groups and which possess an adequatenumber of silane groups in order to form an elastic network. Inaddition, adequate adhesion to various substrates is enabled.Furthermore, silicone-based matrixes may be used. A structure of suchpolymers may be obtained which form microstructures in the crosslinkedcomposition, thereby yielding cross-linked polymer compounds with anexcellent mechanical strength. Furthermore, because of the distributionof the reactive silane groups, a high elasticity may be obtained aftercross-linking.

Exemplary Synthetic Procedures Example 1 Synthesis of CdSe CoreNanocrystals

0.560 g (560 mg) of ODPA solid was added to a 3-neck 25 ml round-bottomflask and 6 g TOPO solid was added to the flask. 0.120 g (120 mg) of CdOsolid was added to the flask. With the flask sealed and the reagentsinside (CdO, ODPA, TOPO), heat the reaction to 120° C. under flowing UHPArgon gas. When the reaction mixture becomes liquid, begin stirring at800 RPM to completely distribute the CdO and ODPA. When the temperatureequilibrates at around 120° C., begin degassing the reaction mixture:Standard degas is for 30 minutes at as low a vacuum as the system canmaintain, preferably between 10-30 torr. After the first degas, switchthe reaction back to flowing UHP Argon gas. The temperature of thereaction was raised to 280° C. to dissociate the CdO. Dissociation isaccompanied by a loss of the typical red color for CdO. Afterdissociation of the CdO, cool the reaction to 120° C. for the 2nddegassing step. Preferably this step is done slowly. In one embodimentthis is done in increments of 40 degrees and allowed to equilibrate ateach step. When the reaction mixture has cooled to about 120° C., beginthe second degassing step. The second degassing is typically 1 hour atthe lowest vacuum level possible. After the second degassing, switch thereaction back to flowing UHP Argon. Heat the reaction mixture. Inject3.0 g TOP into the reaction solution as temperature increases above 280°C. Equilibrate the reaction solution at 370° C. When the reaction isequilibrated at 370° C., inject 0.836 g of 14% Se:TOP stock solutioninto the solution. The reaction is run until the desired visibleemission from the core is achieved. For CdSe cores the time is usuallybetween 0.5 and 10 minutes. To stop the reaction: while continuing tostir and flow UHP Argon through the reaction, rapidly cool the solutionby blowing nitrogen on the outside of the flask. When the reactiontemperature is around 80° C., expose the reaction solution to air andinject approximately 6 mL of toluene. Precipitate the CdSe nanocrystalsthrough the addition of 2-propanol (IPA) to the reaction solutions.Preferably the mixture should be approximately 50/50 (by volume)reaction solution/IPA to achieve the desired precipitation. Centrifugefor 5 minutes at 6000 RPM. Redissolve the CdSe in as little toluene aspossible solid (<2 mL). Precipitate the CdSe again using IPA.Centrifuge. Decant the supernatant liquid. Dissolve the CdSe solid inanhydrous toluene.

Example 2 Synthesis of CdSe/CdS Core-Shell Nanocrystal HeterostructuresHaving PLQY>90%

Transfer 0.290 g (290 mg) of ODPA into a round bottom flask. Transfer0.080 g (80 mg) of hexylphosphonic acid (HPA) into the flask. Transfer 3g TOPO into the flask. Transfer 0.090 g (90 mg) of CdO solid into thereaction flask. With the flask sealed and the reagents inside (CdO,ODPA, TOPO, HPA), heat the reaction to 120° C. under flowing UHP Argongas. When the reaction mixture becomes liquid, at about 60° C., beginstirring at 800 RPM to completely distribute the CdO, ODPA, and HPA.When the temperature settles at 120° C., begin degassing the reactionmixture. After the degas step, switch the reaction back to flowing UHPArgon gas. Raise the temperature of the reaction to 280° C. todissociate the CdO. Increase the temperature set-point of the reactionto 320° C. Inject 1.5 g TOP into the reaction solution as temperatureincreases above 280° C. When the reaction is equilibrated at 320° C.,inject a mixture of 1.447 g of 7.4% S:TOP stock solution and 0.235 gconcentration-adjusted CdSe seed stock into the reaction solution.Immediately reduce the set point of the temperature controller to 300°C. Allow the reaction to proceed for the requisite time to necessary toproduce the desired length and width of shell, yielding a rod having anaspect ratio as between 1.5 and 10, more preferably between 3 and 6.Reaction temperature for shell growth is between 120° C. and 380° C.,preferably between 260° C. and 320° C., more preferably between 290° C.and 300° C.

The reaction is monitored by testing a sample to determine theabsorbance at 400 nm and the at the CdSe exciton peak. Most preferablythe reaction is stopped when the absorbance at 400 nm divided by theabsorbance at the CdSe exciton peak is between about 25-30, but theinvention contemplates that the absorbance ratio may be between about 6and about 100, preferably between about 15-35. By “stopping the growth”it is meant that any method steps may be employed known in the art ifdesired and available to cease the growth of the shell. Some methodswill lead to quicker cessation of shell growth than others.

Absorbance measuring may be performed by UV-VIS spectroscopic analyticalmethod, such as a method including flow injection analysis forcontinuous monitoring of the reaction. In an embodiment, the reaction isstopped or arrested by removing a heating mantle and allowing thereaction vessel to cool. When the reaction temperature is aroundapproximately 80 degrees Celsius, the reaction solution is exposed toair and approximately 4-6 mL of toluene is injected. The quantum dotsare purified by transferring the reaction solution into four smallcentrifuge tubes, so that an equal volume is in each tube. The QDHproduct is precipitated through the addition of 2-propanol (IPA) to thereaction solutions. Following centrifuging, the supernatant liquid isdecanted. The QDH is redissolved in as little toluene as possible (e.g.,less than approximately 2 mL) and re-concentrated into one centrifugetube. The precipitation and centrifugation steps are repeated. The finalsolid product is then dissolved in approximately 2 g of toluene.

Example 3 Synthesis of CdSe/CdS Quantum Dot Having an Absorbance RatioBetween 6-100

A quantum dot was fabricated according to Example 2 and having anabsorbance ratio between 6-100. FIG. 10 is a transmission electronmicroscope (TEM) image 1000 of a sample of core/shell (1002/1004)CdSe/CdS quantum dots, in accordance with an embodiment of the presentinvention. The TEM image 1000 indicates that there are substantially nostructural defects as can be deduced from the low density of stackingfaults and lack of other visible defects along the semiconductorstructure 1002/1004.

Example 4 Synthesis of CdSe/CdS Red Quantum Dot with a PLQY=96%

Quantum dots were fabricated according to Example 2 and having anabsorbance ratio between 6-100, and having a PLQY of 96% at 606 nm. Theaverage length (from TEM data) is 22.3 nm±3.1 nm. The average width(from TEM data) is 6.0 nm±0.6 nm. The average aspect ratio (from TEMdata) is 3.8±0.6. FIG. 11 is a plot 1100 including a UV-Vis absorbancespectrum 1102 and photoluminescent emission spectrum 1104 for a CdSe/CdScore/shell quantum dot having a PLQY of 96%, in accordance with anembodiment of the present invention. The quantum dot has essentially nooverlapping absorption and emission bands. FIG. 12 is a transmissionelectron microscope (TEM) image 1200 of a sample of CdSe/CdS quantumdots 1202 fabricated according to example 4, in accordance with anembodiment of the present invention.

Example 5 Reactive Ligand Exchange for Quantum Dot Structures

0.235 g of concentration-adjusted CdSe stock from Example 2 are exposedto a reactive exchange chemical, trimethylsilylpyrollidine (TMS-Pyr),for 20 minutes in an air-free environment and are mixed completely.After 20 minutes, an alcohol, usually 2-propanol or methanol is added tothe mixture to quench the reactivity of the TMS-Pyr reagent, and toprecipitate the reactively exchanged CdSe particles. The precipitatedparticles are centrifuged at 6000 RPM for 5 minutes. The resultingsupernatant liquid is decanted and the precipitate are re-dissolved in0.235 g of anhydrous toluene for use in the procedure described inExample 2. Reactive ligand exchange is used to introduce any number ofdesired surface functionalities to the surface of quantum dot coresprior to rod growth or the surface of the core/shell particles aftersynthesis.

Example 6 Coating Semiconductor Nanocrystalline Core/Shell Pairing withSilica Using Dioctyl Sodium Sulfosuccinate (AOT)

Approximately 4.5 g of AOT is dissolved in 50 mL of cyclohexane. 0.5 gof QDH is precipitated w/methanol, and then re-dissolved in hexane. 20μL of 3-aminopropyltrimethoxysilane (APTMS) is added and stirred for 30minutes. 900 μL of NH4OH (29 wt %) is added into the solutionimmediately followed by 600 μL of TEOS. The solution is stirred forabout 16 hrs which allows the mixture to react until a silica shellcoats the nanocrystal. The silica coated particles are precipitated byMeOH and the precipitated particles are separated from the supernatantusing a centrifuge. The SiO₂ coated particles can be re-dispersed intoluene or left in cyclohexane.

Example 7 Coating a Semiconductor Nanocrystal with Silica Using IGEPALCO-520

Approximately 4.46 g of Igepal CO-520 (Polyoxyethylene (5)nonylphenylether) is dissolved in 50 mL of cyclohexane and allowed tomix. “n” may be 3, 4, 5, 6, 7, 8, 9 or 10, preferably about 5. 0.5 gramsof quantum dots dissolved in toluene are added. 20 μL of 3-APTMS isadded and stirred for about 30 minutes. 900 μL of NH₄OH (29 wt %) isadded into the solution immediately followed by 600 μL of TEOS. Thesolution is stirred for about 16 hrs at 1600 rpm which allows themixture to react until a silica shell coats the nanocrystal. Themicelles are broken up by IPA and collected using a centrifuge. The SiO₂coated particles may be re-dispersed in toluene or left in cyclohexanefor polymer integration.

Example 8 Methoxy Silane Coupling Agent

Silica-shelled core-shell quantum dots are dispersed in 20 parts tolueneto 1 part (MeO)₃SiR(R=allyl or vinyl), and constantly stirred to allowthe coupling reaction to take place. The functionalized particles areseparated and cleaned by precipitation with IPA and centrifugation at6000 rpm for 10 min. The process is repeated two or more times. Cleanedparticles are dispersed in a known amount of toluene or polymersolution.

Example 9 Quantum Dot/Polymer Preparation

To prepare the films, a known mass of quantum dots in toluene orcyclohexane is added to premade polymer solution, depending on solventcompatibility of the polymer matrix used. Other solvents may also beused for dissolution, if so desired for polarity match with the matrixor to increase or decrease the viscosity or rate of solvent evolutionfrom the cast film.

Example 10 Film Casting

The composite compositions are prepared by drop casting approximately360 μL of QDH polymer solution onto a 12 mm glass round. The amount ofquantum dots added to the polymer solution can be adjusted for differentoptical densities in the final QDH film. After casting films, the slowevaporation of solvent is important to give a film free of large surfaceimperfections. QDH-polymer solutions in toluene are allowed to evaporatein a vented fume hood. The films are cast on a level stainless plate.Once films are dried they are analyzed for PLQY and UV-Vis properties.

Example 11

The surface of silica-shelled quantum dot was functionalized using avariety of methoxy and ethoxy silanes: (MeO)₃SiAllyl, (MeO)₃SiVinyl,(MeO)₂SiMeVinyl, (EtO)₃SiVinyl, EtOSi(Vinyl)₃. The functionalizedsilica-shelled quantum dot was then used in the standard polymerformulation with additives for crosslinking, as well as without anyfurther crosslinking co-agents such as TAIC in the case of EVA ordivinylsilanes for siloxanes.

Example 12

In one embodiment, it is preferred that the olefin group is able toparticipate in a crosslinking process through radical mechanism in thecase of EVA or through hydrosilylation process in the case of siloxanes.Allyl and vinyl are preferred, but other olefins can be included.

Example 13

In one embodiment, the degree of crosslinking may be increased usingquantum dots with a higher density of the olefin groups on silicasurface of quantum dots.

Example 14 Using Polarity

The surface of a silica-shelled particle is modified withorgano-substituted silanes in order to maximize the compatibility with apolymer matrix such as the polysiloxanes for LEDs. The silica surface ismodified with organo-substituted silanes, and its properties aretherefore modified by the grafted functional groups.

Example 15 Pt Catalyst

A platinum-based catalyst may be introduced in Examples 9-14. Inaddition to the functionalized silica particles, two competing orcomplementary catalysts are available for cross-linking.

Example 16 Thiol Catalyst

The Pt catalyst of example 15 is replaced with a thiol catalyst with athiol-ene reaction. Di-thiols or multifunctional thiols are used. Theapproach enables UV curing in place of heat curing.

The above described semiconductor structures, e.g., quantum dots, may befabricated to further include one or more compositional transitionlayers between portions of the structures, e.g., between core and shellportions. Inclusion of such a transition layer may reduce or eliminateany performance inefficiency associated with otherwise abrupt junctionsbetween the different portions of the structures. For example, theinclusion of a compositional transition layer may be used to suppressAuger recombination within a quantum dot structure. Auger recombinationevents translate to energy from one exciton being non-radiativelytransferred to another charge carrier. Such recombination in quantumdots typically occurs on sub-nanosecond time scales such that a veryshort multi-exciton lifetime indicates non-radiative recombination,while higher nanosecond bi-exciton lifetimes indicate radiativerecombination. A radiative bi-exciton has a lifetime approximately 2-4times shorter than radiative single exciton.

Temperature performance can be highly dependent on semiconductor surfacepreparation, indicating that accessibility of surface states may beresponsible for high temperature behavior. Light-induced intermittencyis a feature of quantum dots which is believed to lead to poorperformance at high flux intensities. The phenomenon is relativelytemperature independent, which leads to a surface state insensitivemodel for the behavior. Instead, it appears to be controlled primarilyby the composition and nature of the internal particle interfaces.

Accordingly, in an embodiment, a quantum dot architecture is fabricatedto include one or more of a graded composition, multiple layers, andasymmetry to control exciton dynamics. Such tailoring of the quantum dotarchitecture may be performed to provide optimum photoluminescent (PL)quantum yield performance at high temperatures (e.g., up toapproximately 200 degrees Celsius) and high incident flux intensities(e.g., up to approximately 150 W/cm²). As a benchmark, a variety ofarchitectures for quantum dots have been shown to provide very highquantum yields (e.g., 80% or higher) at room temperatures, but oftenexhibit temperature droop and photobleaching at high temperatures andhigh fluxes. One or more embodiments of the present invention aredirected to the fabrication of quantum dots having compositions andarchitectures suitable to address the above described structural issueswhich have been known to cause temperature droop and photobleaching.

More specifically, as is described in greater detail below inassociation with FIGS. 13-15, an optimal particle (e.g., quantum dotstructure) may have one or more of a high aspect ratio, a large volumerelative to other quantum dot heterostructures, and graded or alloyedtransitions between different semiconductor materials. The graded oralloyed transitions can be used to render a compositional and structuraltransition from one component (such as a quantum dot core) to anothercomponent (such as a quantum dot shell) a smooth function rather than astep function. In one embodiment, the terms “graded,” “gradient,” or“grading” are used to convey gradual transitioning from onesemiconductor to another. In one embodiment, the terms “alloy,”“alloyed,” or “alloying” are used to convey an entire volume having afixed intermediate composition. In more specific embodiments, core orseed volume is maximized relative to shell volume for a given emissioncolor. A graded or alloyed core/shell transition layer may be includedbetween the two volumes.

Furthermore, temperature droop may be attributed to surface states whichare made more accessible to an excited state at high temperatures. In anembodiment, temperature droop is mitigated or eliminated by using large,crystalline volume in addition to a large bandgap crystalline coatingover the core and an amorphous outer coating applied to the resulting tothe particle. In an embodiment, photo-bleaching is mitigated oreliminated by alloying or grading a well-passivated particle attransitions between semiconductor layers. Alloying or grading thetransition between different semiconductors may provide certainty formomentum of the carriers, reducing non-radiative excited staterelaxation processes.

Past approaches to addressing quantum dot inefficiencies have includedthe use of spherical alloyed particles which demonstrate goodphotobleaching characteristics, but with starting PLQYs in the 40%range. Such systems provide reasonable behavior at high intensity butwith emission efficiencies that may be too low to be useful in a devicesuch as a lighting device. Furthermore, past approaches have noteffectively addressed both photo-bleaching and temperature droop in asame particle.

In an exemplary embodiment, a quantum dot is fabricated to include coresemiconductor nanoparticle is fabricated to have an aspect ratio between1 and 2, a graded transition of ½ to 2 monolayers between core andshell, a rod-shaped shell with a high aspect ratio (e.g., between 4 and6), a graded transition of ½ to 2 monolayers between first and secondsemiconductor shell materials, a final intrinsic layer of the secondsemiconductor shell material, and an outer coating of amorphousinsulator, such as SiO₂. Such quantum dots exhibit very high PLQYspreserved both at room temperature and under conditions of hightemperature and high flux for long periods of time. More generally, suchquantum dots may provide optimal temperature stability and hightemperature performance by stabilizing a seed/rod structure with acoating of amorphous wide bandgap insulator. The addition of theinsulating layer may serve to passivate the surface of the quantum dots,which may be a key to improving reliability. Additional insulatorcoating may be included to provide extremely high room temperaturequantum yields, as well as isolation of an electronically active corefrom its surroundings, excellent temperature and light stability, and apathway to a robust and stable composite with relevant plastics for LEDapplications. Essentially, in an embodiment, the insulator provides anextra barrier between the active quantum dot and the matrix, preventingoxidation of the surface, uncontrolled alloying, ligand loss and otherundesirable high temperature processes. Since surface defects can bedetrimental to room-temperature and high temperature performance ofquantum dots, the addition of an amorphous layer can be key to theirdurability in the LED product. The above and other examples ofarchitectures for quantum dots having one or more alloyed or gradedcompositional transition layer are illustrated in and described below inassociation with FIGS. 13-15.

In a first example, FIG. 13 illustrates a cross-sectional view of asemiconductor structure having a nanocrystalline core andnanocrystalline shell pairing with one compositional transition layer,in accordance with an embodiment of the present invention.

Referring to FIG. 13, a semiconductor structure 1300 includes ananocrystalline core 1302 composed of a first semiconductor material. Ananocrystalline shell 1304 composed of a second, different,semiconductor material at least partially surrounds the nanocrystallinecore 1302. A compositional transition layer 1310 is disposed between,and in contact with, the nanocrystalline core 1302 and nanocrystallineshell 1304. The compositional transition layer 1310 has a compositionintermediate to the first and second semiconductor materials.

In an embodiment, the compositional transition layer 1310 is an alloyedlayer composed of a mixture of the first and second semiconductormaterials. In another embodiment, the compositional transition layer1310 is a graded layer composed of a compositional gradient of the firstsemiconductor material proximate to the nanocrystalline core 1302through to the second semiconductor material proximate to thenanocrystalline shell 1304. In either case, in a specific embodiment,the compositional transition layer 1310 has a thickness approximately inthe range of 1.5-2 monolayers. Exemplary embodiments include a structure1300 where the first semiconductor material is cadmium selenide (CdSe),the second semiconductor material is cadmium sulfide (CdS), and thecompositional transition layer 1310 is composed of CdSe_(x)S_(y), where0<x<1 and 0<y<1, or where the first semiconductor material is cadmiumselenide (CdSe), the second semiconductor material is zinc selenide(ZnSe), and the compositional transition layer 1310 is composed ofCd_(x)Zn_(y)Se, where 0<x<1 and 0<y<1.

In accordance with an embodiment of the present invention, thecompositional transition layer 1310 passivates or reduces trap stateswhere the nanocrystalline shell 1304 surrounds the nanocrystalline core1302. Exemplary embodiments of core and/or shell parameters include astructure 1300 where the nanocrystalline core 1302 is an anisotropicnanocrystalline core having an aspect ratio between, but not including,1.0 and 2.0 (in a specific embodiment, approximately in the range of1.01-1.2), and the nanocrystalline shell is an anisotropicnanocrystalline shell having an aspect ratio approximately in the rangeof 4-6.

In an embodiment, the nanocrystalline shell 1304 completely surroundsthe nanocrystalline core 1302, as depicted in FIG. 13. In an alternativeembodiment, however, the nanocrystalline shell 1304 only partiallysurrounds the nanocrystalline core 1302, exposing a portion of thenanocrystalline core 1302. Furthermore, in either case, thenanocrystalline core 1302 may be disposed in an asymmetric orientationwith respect to the nanocrystalline shell 1304. In one or moreembodiments, semiconductor structures such as 1300 are fabricated tofurther include a nanocrystalline outer shell 1306 at least partiallysurrounding the nanocrystalline shell 1304. The nanocrystalline outershell 1306 may be composed of a third semiconductor material differentfrom the first and second semiconductor materials, i.e., different fromthe materials of the core 1302 and shell 1304. The nanocrystalline outershell 1306 may completely surround the nanocrystalline shell 1304 or mayonly partially surround the nanocrystalline shell 1304, exposing aportion of the nanocrystalline shell 1304.

For embodiments including a nanocrystalline outer shell, an additionalcompositional transition layer may be included. Thus, in a secondexample, FIG. 14 illustrates a cross-sectional view of a semiconductorstructure having a nanocrystalline core/nanocrystallineshell/nanocrystalline outer shell combination with two compositionaltransition layers, in accordance with an embodiment of the presentinvention.

Referring to FIG. 14, a semiconductor structure 1400 includes thenanocrystalline core 1302, nanocrystalline shell 1304, nanocrystallineouter shell 1306 and compositional transition layer 1310 of structure1300. However, in addition, semiconductor structure 1400 includes asecond compositional transition layer 1412 disposed between, and incontact with, the nanocrystalline shell 1304 and the nanocrystallineouter shell 1306. The second compositional transition layer 1412 has acomposition intermediate to the second and third semiconductormaterials, i.e., intermediate to the semiconductor materials of theshell 1304 and outer shell 1306.

In an embodiment, the second compositional transition layer 1412 is analloyed layer composed of a mixture of the second and thirdsemiconductor materials. In another embodiment, the second compositionaltransition layer 1412 is a graded layer composed of a compositionalgradient of the second semiconductor material proximate to thenanocrystalline shell 1304 through to the third semiconductor materialproximate to the nanocrystalline outer shell 1306. In either case, in aspecific embodiment, the second compositional transition layer 1412 hasa thickness approximately in the range of 1.5-2 monolayers. Exemplaryembodiments include a structure 1400 where the first semiconductormaterial is cadmium selenide (CdSe), the second semiconductor materialis cadmium sulfide (CdS), the third semiconductor material is zincsulfide (ZnS), and the second compositional transition layer 1412 iscomposed of Cd_(x)Zn_(y)S, where 0<x<1 and 0<y<1, or the firstsemiconductor material is cadmium selenide (CdSe), the secondsemiconductor material is zinc selenide (ZnSe), the third semiconductormaterial is zinc sulfide (ZnS), and the second compositional transitionlayer 1412 is composed of ZnSe_(x)S_(y), where 0<x<1 and 0<y<1. Inaccordance with an embodiment of the present invention, the secondcompositional transition layer 1412 passivates or reduces trap stateswhere the nanocrystalline outer shell 1306 surrounds the nanocrystallineshell 1304.

For other embodiments including a nanocrystalline outer shell, an outercompositional transition layer may be included without including aninner compositional transition layer. Thus, in a third example, FIG. 15illustrates a cross-sectional view of a semiconductor structure having ananocrystalline core/nanocrystalline shell/nanocrystalline outer shellcombination with one compositional transition layer, in accordance withan embodiment of the present invention.

Referring to FIG. 15, a semiconductor structure 1500 includes thenanocrystalline core 1302, nanocrystalline shell 1304, andnanocrystalline outer shell 1306 of structure 1300. In addition, thesemiconductor structure 1500 includes the compositional transition layer1412 of structure 1400 disposed between, and in contact with, thenanocrystalline shell 1304 and the nanocrystalline outer shell 1306.However, structure 1500 does not include the compositional transitionlayer 1310 of structure 1300, i.e., there is no compositional transitionlayer between the core 1302 and shell 1304.

Referring to FIGS. 13-15, as depicted, the structures 1300, 1400 and15500 may further include an insulator coating 1308 surrounding andencapsulating the nanocrystalline core/nanocrystalline shell pairing ornanocrystalline core/nanocrystalline shell/nanocrystalline outer shellcombination. In one such embodiment, the insulator coating is composedof an amorphous material such as, but not limited to, silicon dioxide(SiO₂), silicon oxide (SiO_(x)), aluminum oxide (Al₂O₃), zirconia(ZrO_(x)), titania (TiO_(x)), or hafnia (HfO_(x)). In an embodiment, thestructures 1300, 1400 and 1500 are quantum dot structures. For example,structures 1300, 1400 and 1500 may be used as a down-converting quantumdot or an up-shifting quantum dot.

A plurality of quantum dot structures such as structures 1300, 1400 or1500 may be included in a composite having a matrix material and aplurality of the semiconductor structures embedded in the matrixmaterial. In one such embodiment, each of the plurality of semiconductorstructures is cross-linked with, polarity bound by, or tethered to thematrix material. In another such embodiment, each of the plurality ofsemiconductor structures is bound to the matrix material by a covalent,dative, or ionic bond. In either case, in a specific embodiment, one ormore of the semiconductor structures further includes a coupling agentcovalently bonded to an outer surface of the amorphous insulator coating1308.

In another aspect, generally, individual quantum dots exhibitcharacteristic “on/off” events, termed blinking, which representemission from neutral and charged species, respectively. It is thisbehavior that is believed to be responsible for the photobleachingbehavior often observed in quantum dots. The formation of chargedspecies is believed to arise from charge transfer from within thequantum dot to trap states at the core/shell interface, its surface, orthe matrix, and increases as a function of incident intensity. Corevolume and surface have been shown to be a significant parameter inblinking behavior. Of even greater importance may be the formation ofsmooth transitions between different epitaxial semiconductor layers.

Furthermore, changes to the core or seed size and overall dimensions ofthe nanocrystalline semiconductor particle may affect intrinsic particleperformance and stability, and it is important to understand the changesin exciton dynamics in order to prevent or correct detrimental changes.Time resolved optical interrogation techniques may thus be used as partof the characterization process when changes to a quantum dot structureare made. Time resolved photoluminescence (TRPL) provides the excitedstate lifetime, as both a function of incident intensity andtemperature. Transient absorption spectroscopy measures the change inthe absorption coefficient of a sample after it is excited by a short(<1 ps) pulse at 400-450 nm. By monitoring the time-dependence of thischange at different wavelengths (400 nm to 1200 nm) the radiative andnon-radiative dynamics of the decay pathways can be investigated.

Another characterization technique for analyzing high intensity behavioris PLQY at high flux and high temperature. By analyzingphotoluminescence quantum yield at high flux and high temperature, themulti-exciton radiative efficiency and activation energy of trap-statescan be ascertained. An example of time resolved photoluminescence ofnon-alloyed rod materials is given in FIG. 16. FIG. 16 is a plot 1600 ofincreasing photoluminescence (PL) intensity as a function of time(seconds), based on equation 1602 and data plot 1604, for non-alloyedrod materials. The time resolved photoluminescence is obtained at highfluence, e.g., 0.016 and 0.05 mW, with a radiative lifetime ofapproximately 13-14 nanoseconds. As the incident flux is increased by 2orders of magnitude, a fast lifetime competes with regular lifetime, andregular lifetime decreases to 5 nanoseconds. A shorter radiativelifetime is better at high fluxes to reduce bi-exciton population. Oneor more embodiments described herein, such as structures 1300, 1400 or1500, described above, provide quantum dot architectures suitable formulti-exciton radiative efficiency, e.g., for up-shifting ordown-converting layers in lighting devices.

Thus, semiconductor structures having a nanocrystalline core andnanocrystalline shell pairing with one or more compositional transitionlayers have been disclosed.

What is claimed is:
 1. A semiconductor structure, comprising: ananocrystalline core comprising a first semiconductor material; ananocrystalline shell comprising a second, different, semiconductormaterial at least partially surrounding the nanocrystalline core; afirst compositional transition layer disposed between, and in contactwith, the nanocrystalline core and nanocrystalline shell, the firstcompositional transition layer having a composition intermediate to thefirst and second semiconductor materials; a nanocrystalline outer shellat least partially surrounding the nanocrystalline shell, thenanocrystalline outer shell comprising a third semiconductor materialdifferent from the first and second semiconductor materials; and asecond compositional transition layer disposed between, and in contactwith, the nanocrystalline shell and the nanocrystalline outer shell, thesecond compositional transition layer having a composition intermediateto the second and third semiconductor materials.
 2. The semiconductorstructure of claim 1, wherein the first compositional transition layeris an alloyed layer comprising a mixture of the first and secondsemiconductor materials.
 3. The semiconductor structure of claim 1,wherein the first compositional transition layer is a graded layercomprising a compositional gradient of the first semiconductor materialproximate to the nanocrystalline core through to the secondsemiconductor material proximate to the nanocrystalline shell.
 4. Thesemiconductor structure of claim 1, wherein the first compositionaltransition layer has a thickness approximately in the range of 1.5-2monolayers.
 5. The semiconductor structure of claim 1, wherein the firstsemiconductor material is cadmium selenide (CdSe), the secondsemiconductor material is cadmium sulfide (CdS), and the firstcompositional transition layer comprises CdSe_(x)S_(y), where 0<x<1 and0<y<1.
 6. The semiconductor structure of claim 1, wherein the firstsemiconductor material is cadmium selenide (CdSe), the secondsemiconductor material is zinc selenide (ZnSe), and the firstcompositional transition layer comprises Cd_(x)Zn_(y)Se, where 0<x<1 and0<y<1.
 7. The semiconductor structure of claim 1, wherein the firstcompositional transition layer passivates or reduces trap states wherethe nanocrystalline shell surrounds the nanocrystalline core.
 8. Thesemiconductor structure of claim 1, wherein the second compositionaltransition layer is an alloyed layer comprising a mixture of the secondand third semiconductor materials.
 9. The semiconductor structure ofclaim 1, wherein the second compositional transition layer is a gradedlayer comprising a compositional gradient of the second semiconductormaterial proximate to the nanocrystalline shell through to the thirdsemiconductor material proximate to the nanocrystalline outer shell. 10.The semiconductor structure of claim 1, wherein the second compositionaltransition layer has a thickness approximately in the range of 1.5-2monolayers.
 11. The semiconductor structure of claim 1, wherein thefirst semiconductor material is cadmium selenide (CdSe), the secondsemiconductor material is cadmium sulfide (CdS), the third semiconductormaterial is zinc sulfide (ZnS), and the second compositional transitionlayer comprises Cd_(x)Zn_(y)S, where 0<x<1 and 0<y<1.
 12. Thesemiconductor structure of claim 1, wherein the first semiconductormaterial is cadmium selenide (CdSe), the second semiconductor materialis zinc selenide (ZnSe), the third semiconductor material is zincsulfide (ZnS), and the second compositional transition layer comprisesZnSe_(x)S_(y), where 0<x<1 and 0<y<1.
 13. The semiconductor structure ofclaim 1, wherein the second compositional transition layer passivates orreduces trap states where the nanocrystalline outer shell surrounds thenanocrystalline shell.
 14. The semiconductor structure of claim 1,wherein the nanocrystalline core is an anisotropic nanocrystalline corehaving an aspect ratio between, but not including, 1.0 and 2.0.
 15. Thesemiconductor structure of claim 14, wherein the nanocrystalline shellis an anisotropic nanocrystalline shell having an aspect ratioapproximately in the range of 4-6.
 16. The semiconductor structure ofclaim 14, wherein the aspect ratio of the anisotropic nanocrystallinecore is approximately in the range of 1.01-1.2.
 17. The semiconductorstructure of claim 1, further comprising: an insulator coatingsurrounding and encapsulating the nanocrystalline core/nanocrystallineshell/nanocrystalline outer shell structure.
 18. The semiconductorstructure of claim 17, wherein the insulator coating comprises anamorphous material selected from the group consisting of silicon dioxide(SiO₂), silicon oxide (SiO_(x)), aluminum oxide (Al₂O₃), zirconia(ZrO_(x)), titania (TiO_(x)), and hafnia (HfO_(x)).
 19. Thesemiconductor structure of claim 1, wherein the nanocrystalline shellcompletely surrounds the nanocrystalline core.
 20. The semiconductorstructure of claim 1, wherein the nanocrystalline shell only partiallysurrounds the nanocrystalline core, exposing a portion of thenanocrystalline core.
 21. The semiconductor structure of claim 1,wherein the nanocrystalline core is disposed in an asymmetricorientation with respect to the nanocrystalline shell.
 22. Thesemiconductor structure of claim 1, wherein the semiconductor structureis a quantum dot.
 23. The semiconductor structure of claim 22, whereinthe quantum dot is a down-converting quantum dot.
 24. The semiconductorstructure of claim 22, wherein the quantum dot is an up-shifting quantumdot.